The figure of merit in thin film transistors (TFTs) is defined by μV/L2 where μ is the mobility, V is the voltage and L is the gate length. A major problem is partially remedied by the recent advance in metal oxide semiconductor materials in which mobility as high as 80 cm2/V-sec has been demonstrated. However, the metal oxide semiconductor material presently in use is amorphous and must remain amorphous for uniformity and/or reproducibility in the manufacture of multiple devices. Devices produced at low temperatures in many instances are not stable, exhibiting threshold shift under stress. Also, the amorphous metal oxide semiconductor material is physically unstable in that it has a tendency to change to polycrystalline material when heat is applied for a relatively long period of time. This physical stability can be improved by annealing at high temperature for short periods of time.
In the case of the amorphous metal oxide semiconductor material layer, the devices can be produced with more uniformity but the mobility may be less. Performance and physical stability can be increased by annealing. The mobility of TFTs, for example, and the subthreshold slope can also be improved by annealing. This is due to either or both the mobility of the amorphous metal oxide semiconductor or the gate dielectric/metal oxide interface improvement by the high temperature annealing. High temperature annealing reduces traps in the semiconductor/dielectric interface and in the semiconductor material itself, therefore, improving operating stability. For purposes of this disclosure, “operating stability” is defined in terms of the threshold voltage of the TFT, which should remain constant or stable throughout the lifetime. However, high temperature annealing can produce damage in the semiconductor device, i.e. to flexible substrates and/or other plastic layers in the device.
In many applications TFTs are formed on a substrate such as plastic, glass, polymer layers on glass (such as color filters), etc. (hereinafter referred to generically as a temperature sensitive substrate formation) which can only sustain a temperature of approximately 200° C. or less. In such applications it has been proposed that a pulsed ultraviolet (UV) laser provide the required heat. Metal oxide has a very large bandgap and can only absorb energy directly in the deep UV band. There are two major problems with this type of annealing procedure. One problem is that the temperatures can still be raised because of residual temperature absorption of the substrate, and temperature sensitive substrates, such as flexible or plastic substrates or other plastic layers such as color filters, will melt at this temperature. A second problem is that UV lasers are extremely expensive, causing this annealing method to be very expensive. It would be highly desirable to devise a method of low temperature annealing amorphous metal oxide semiconductor material in which the resulting grain sizes are small enough (i.e. remain amorphous) to improve the mobility as well as the reliability and uniformity of multiple TFTs formed therein.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved method of annealing an amorphous metal oxide semiconductor device on a temperature sensitive substrate.
It is another object of the present invention to provide a new and improved method of annealing metal oxide devices on temperature sensitive substrate formations that is low cost and easy to perform.
It is another object of the present invention to provide a new and improved method of annealing metal oxide devices on temperature sensitive substrate formations that improves the performance, reliability and uniformity of the semiconductor devices.